Generally, a constant voltage source for current measurements and a constant current source for voltage measurements have both been utilized to measure "current-voltage characteristics" of a device under test (DUT) such as a semiconductor element. U.S. Pat. No. 4,467,275 has been assigned to the same assignee of an application which has been also published as Japanese Laid-Open Patent Publication No. 59(1983)-75073. The '275 patent discloses a signal source/measurement unit or a stimulus measurement unit (SMU) which can alternatively act as a voltage source/a current monitor or as a current source/a voltage monitor. By utilizing a SMU, a device under test can be characterized without changing external connections, and resulting data can be manipulated and displayed in a wide variety of graphical or tabular forms.
Conventionally, a current flowing through a DUT or a voltage across the DUT (which is generally called "load"), has been measured by applying a known value of either voltage from the voltage source or current from the current source. By using a SMU, the voltage source function is performed to measure a current output through a DUT, or the current source function is performed to measure a voltage output thereacross. Hereafter, an output voltage refers to a known voltage value outputted by the voltage source or for the SMU application, a corresponding voltage output value when the current flows through the DUT. An output current refers to the current flowing through the DUT or for the SMU application, a corresponding current flowing through the DUT when a known voltage is applied from the SMU. Thus, a single output characteristic of the DUT (e.g., voltage versus current) is measured. By varying (sweeping) such applied voltage or current over a predetermined range, the output characteristics of the DUT over the sweeping range are obtained.
In addition, such a voltage source and/or current source is provided with an output restriction function to protect the DUT in case of a failure. When a current flowing through the load during an application of voltage reaches a predetermined current limiting value of the voltage source, the output restriction function does not allow an excessive amount of current to flow through the load. Thus, it substantially serves as a constant current source. Similarly, when an output voltage during an application of current reaches the output voltage limiting value of the current source, the output restriction function does not allow an excess amount of voltage. Thus, it substantially serves as a constant voltage source.
FIG. 1A shows the current-voltage characteristics of a load resistance, when they are conventionally measured by using a voltage source, wherein the output current limiting value of the current source is fixed at I.sub.L, and a given voltage output is swept from V.sub.1 to V.sub.10. At each corresponding voltage output current flowing through the load resistance is changed, accordingly from I.sub.1 to I.sub.L. When an output of the voltage source varies from V.sub.9 to V.sub.10, the current flowing through the load resistance reaches a given output current limiting value of the voltage source, I.sub.L. Thus, the current flowing through the load would not exceed I.sub.L.
On the other hand, FIG. 1B shows the current-voltage characteristics of the load resistance measured by using a current source. The output voltage limiting value of the current source is fixed at V.sub.L, and the current sweeps from I.sub.1 to I.sub.10 in order. At each corresponding output current, the voltage across the load resistance is varied, proceeding from V.sub.1 to V.sub.L. When the output of the current source is changed from I.sub.9 to I.sub.10, the output voltage reaches the output voltage limiting value, V.sub.L, so that the voltage across the load resistance is limited to V.sub.L.
However, these conventional methods have several problems as-will be described below. The problems become apparent especially when a rapid change in the output characteristics of the load is measured, such as the breakdown region of a diode. FIGS. 2A and 2B show a voltage which has been swept from V.sub.1 through V.sub.13 and a current flowing through the load at each voltage. FIG. 2A shows a rapid change in the current flow through the load in response to changing the output voltage applied to the load between V.sub.10 and V.sub.11. Such a rapid change in current is not seen, i.e., the monitored characteristic is very imprecise. Even when the output voltage (sweep voltage) V.sub.10 falls between Va and Vb as shown in FIG. 2B, a precise measurement of such a rapid change cannot be accomplished as a huge change in the current output of a voltage source occurs over a very short range of the applied voltage.
In order to solve the above-described problems, two conventional approaches have been attempted as follows: One approach is to reduce the amount of increment in the voltage output (sweep voltage step). This approach may be effective for a measurement of the output characteristics of a load whose voltage output region has at least one rapid change. For example, the load as shown in FIG. 2A has one rapid change between V.sub.10 and V.sub.11. However, the overall efficiency of the measurements is sacrificed. That is, this would require, for instance, an extensive sweep time and consume a great amount of memory in the measuring device.
The other approach is to utilize a current source in place of a voltage source. Although this would allow the measurement of the output characteristics of a load with a rapid change in its current output, a rapid change in its voltage output, e.g., from V.sub.1 to V.sub.10 still cannot be precisely determined.
The object of the present invention is, therefore, to overcome the above-described problems of the conventional methods, and to provide a method of more accurately and efficiently measuring the current-voltage characteristics of a DUT. Another object is to provide a method of more precisely measuring a rapid change in the output characteristics of the DUT.